This invention relates to an improvement of a current sensing circuit which measures current from the voltage drop across the two terminals of a current sensing resistor, and in particular to a current sensing circuit provided with a circuit to automatically compensate for the offset voltage of a difference amplifier which amplifies the current sensing resistor voltage.
A current sensing circuit measures current from the voltage drop developed across the two terminals of a current sensing resistor. This can be done because the voltage drop across the current sensing resistor is proportional to the current. The voltage drop across the two terminals of the current sensing resistor is the product of the current sensing resistor value and the current. Since the current sensing resistor is connected in series with the load, it is desirable for the resistor value to be as small as possible. This is because electric power consumed by the current sensing resistor is wasted power. Since electric power consumed by the current sensing resistor increases in proportion to the resistor value, the current sensing resistor value should be as small as possible to reduce power consumption.
However, as the value of the current sensing resistor is reduced, the voltage drop across the two terminals of the resistor also decreases. Therefore, it is necessary to amplify the voltage drop across the current sensing resistor with a difference amplifier. In principle, if the voltage across the current sensing resistor is amplified by a difference amplifier under ideal conditions, accurate current measurement should be possible when the current sensing resistor value has been reduced significantly. However, The difference amplifier produces a non-zero output voltage even when the input voltage is zero. This is the so-called "offset voltage" of the difference amplifier. For example, if a difference amplifier with an input referred offset voltage of 10 mV is used to amplify a voltage signal with a maximum input voltage of 100 mV, the offset voltage reaches 10% of the maximum input voltage. The offset voltage is the cause of current sensing circuit error.
A current sensing circuit with difference amplifier offset voltage compensation for accurate current measurement is cited in Japanese Non-examined Patent Publication No.7-77546 issued on Mar. 20, 1995. Turning to FIG. 1, the current sensing circuit cited in this disclosure is shown. In the current sensing circuit of FIG. 1, the output of difference amplifier 1 is converted to a digital value by analog to digital (A/D) converter 2. A switching device 4 is connected in series with current sensing resistor 3. The output of A/D converter 2 when the switching device 4 is in the off state, namely when current in the current sensing resistor is zero, is used as the zero-point for current measurement. For example, if the A/D converter 2 output voltage is 0.5V when current through the current sensing resistor 3 is zero, current measurements are made with a 0.5V output voltage used as the reference voltage. Current is thereby measured with difference amplifier 1 offset voltage compensation.
The current sensing circuit of FIG. 1 has the characteristic that the effect of difference amplifier 1 offset voltage is eliminated and current can be accurately measured. However, this circuit has the problem that it cannot utilize 100% of the difference amplifier 1 output voltage range for current measurement and measurement accuracy is thereby reduced. Specifically, when difference amplifier 1 offset voltage is considered, the output voltage range is reduced. In the circuit described in this disclosure, the current sensing resistor 3 value is made small to reduce resistive power loss. As a result, the voltage drop developed across the current sensing resistor 3 is small and the effect of offset voltage increases. For example, a current sensing circuit using a 0.1 W current sensing resistor 3 for measuring 0A to 0.5A currents develops a maximum voltage of 50 mV across the current sensing resistor 3. In this case, if the difference amplifier 1 has a .+-.15 mV offset voltage and can linearly amplify a 50 mV input voltage, the situation is as shown in FIG. 2. Here the output voltage range which is usable for current sensing is limited to 50 mV-(15.times.2)mV, or only 20 mV. The difference amplifier 1 can amplify linearly in the range from 30 mV to 50 mV and this range can be used for current measurement. Therefore, since the range of voltages input to the difference amplifier 1 is limited, and since this range is used to sense currents in the range of 0A to 0.5A, the current sensing circuit of FIG. 1 has the drawback that current cannot be measured with high accuracy.
Further, since the current sensing circuit of FIG. 1 uses microcomputer 5 to compensate for offset voltage from difference amplifier 1 output and calculate current, it also has the drawback that microcomputer 5 computation is time consuming. This is because each time the microcomputer 5 calculates a current value, it compensates for offset voltage.
The present invention was developed with the object of overcoming these types of drawbacks. Thus a primary object of the present invention is to provide a current sensing circuit with automatic offset compensation which can measure current accurately with high precision and which can calculate current values with simple and fast computations.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.